Short answer is that we don't assume that. Not sure if you've heard of the fermi paradox but the solutions proposed to that seem pretty relevant to what you're asking here.

Long answer is simply that we don't think that but that our discussions typically revolve around advanced civs because they pose the most interesting and relevant discussions to our world today. We only talk about more advanced civs because the types of aliens relevant to our own interests and security would probably be advanced. A hunter-gatherer civilization probably wouldn't be a threat, we wouldn't be able to communicate with them from long distance, and they wouldn't visit us here on earth so we just don't talk about them as much. Scientifically speaking, aliens could be anything, but we just don't know. Your discussion above is one of many possible conditions of extraterrestrial life.